FIG. 1 shows a single-line distribution feeder 10, comprising a distribution main line 12 and two lateral lines 14 and 16 operating into loads 18 and 20. There may of course be additional lateral lines and their loads off of main line 12. Distribution feeder 10 is fed by a source 22, with main line 12 being protected by a relay 24 and a circuit breaker 26. The lateral lines are typically protected by fuses 28. Fuses 28 are a popular choice for protection of lateral lines for many utilities because of their relatively low cost, although they do have to be replaced following operation.
Relay 24 will trip breaker 26 in response to all main line faults, thereby interrupting power to all the main line and lateral loads, but also serves as backup protection for faults on the various lateral circuits. Typical operational coordination between relay 24 and the fuses on the various lateral lines requires that the relay time-to-trip be longer than the time-to-trip characteristic of the power fuses on the lateral lines. Although this in itself is a compromise on the part of the relay operation which results in some voltage "sag" in the area surrounding the faulted feeder when there is a fault on a lateral, it is typically an acceptable compromise to the utility and its customers.
However, in the case where the fault evolves from a phase-to-phase to ground fault to a phase-to-ground fault, the coordination with the relay becomes more complex and difficult. It is preferable, of course, that faults on individual lateral lines be cleared by the fuse(s) present on that lateral. This means that the fuse should operate before the relay does. It is also important, however, that main line faults be recognized and cleared as promptly as possible. When a fault evolves, the relay "times" toward a trip from the beginning of the fault. Setting the trip time longer so as to accommodate a fuse operating to protect one phase in an evolving fault will severely compromise the main line clearing function of the relay.
In particular, such an increase in the relay trip time will result in main line faults persisting longer than they otherwise need to, resulting in a voltage sag to the surrounding distribution lines for a greater time than necessary, which is undesirable. The alternative to date to increasing the relay trip time, however, is to allow the main line relay to time from the first indication of a fault and then trip the main line breaker before the protective element (the fuse) for the second part of the evolving fault operates, which results in a power outage to all the customers on the main line for a time instead of just for those customers on the lateral on which the fault occurs.
Hence, there is at present a mis-coordination problem for evolving faults between a protective relay and downstream protective elements, resulting in undesirable consequences for the power system customers, regardless of whether the relay time is increased to permit the downstream elements, such as a fuse, to operate to clear the evolving fault or to maintain the relay trip time at a value where it operates before the downstream element.